The invention relates to the handover of a mobile station and especially to a quick handover from one cell to another in a micro-cellular network when the signal strength drops suddenly.
It is typical of cellular mobile systems that mobile stations can freely move and connect from one cell to another within the area of the mobile system. Crossover is just a re-registration to a new cell, when the mobile station does not have an ongoing call. When the mobile station does have an ongoing call during crossover, the call must also be connected from one base station to another with as little disturbance as possible. A crossover performed during a call is called handover. Handover may also take place within the cell from one traffic channel to another. To detect the need for handover in a mobile system and select the appropriate target cell, various measurements of connection quality are required as well as measurement of the field strengths of the adjacent cells. Handover from a serving cell to an adjacent cell may take place, for instance when the measurement results of the mobile/base station show a low signal level or quality in the current serving cell and a better signal level and/or quality is available in an adjacent cell. To ensure the stability of the mobile network, the measurement results and parameters used in handover are averaged within a certain time period. Thus handover becomes less susceptible to distorted measurement results caused by momentary interference or fading.
In urban-like conditions and especially in a network using microcells, the solution described above does not, however, react fast enough to a sudden drop in signal strength of the serving cell, for instance when a fast moving mobile station suddenly moves behind an obstacle in the terrain or a building. The measurement results are not real-time because they are averaged and there is a delay in reporting. The most suitable cell according to the latest measurement results may suddenly be obscured by an obstacle, which means that the signal it offers is no longer sufficient to maintain the call.
Finnish patent application FI 960857 provides one solution to obtaining real-time measurement results. The application describes a handover method where a real-time uplink signal measurement of the mobile station is initialized in the neighbouring base stations, when the level or quality of the signal in the serving base station has dropped below a predefined trigger value. The base station with the most suitable measurement values is then selected for handover, when the measured uplink signal value of the serving base station has dropped below a specific threshold value for handover.
The problem in the solution described above is that it is based on a predefined constant of the signal level and/or quality, which is the same indoors and outdoors. It is typical of a network using micro-cells, in particular, that the level and quality of the signal is extremely high outdoors and relatively low inside buildings. Thus in the above solution, a quick drop in the level or quality of the signal outdoors may go unnoticed, as the level and/or quality remains satisfactory and higher than the trigger value, even though a neighbouring base station could offer a better signal. Indoors the level and/or quality of the signal may continuously be below the trigger value causing a continuous real-time measuring of the neighbouring base stations, even though none of the base stations can provide a better signal.
It is an object of the invention to provide a method and an apparatus implementing the method to solve the above-mentioned problems. The object is achieved by a method characterized by setting a predefined signal change as the trigger value, by calculating the change in the level and/or quality of the signal, and if the change is at least equal to the trigger value, by executing a first handover algorithm.
The term handover algorithm refers herein to the actions, calculations and comparisons including the handover criteria, which the operator has specified and according to which it is possible to decide when to handover, if it becomes necessary.
Another object of the invention is a mobile system where the method of the invention can be used. The mobile system is characterized in that it comprises calculation means to calculate the change in the level and/or quality of the signal, comparison means to compare the change in the signal level with a predefined trigger value, and execution means responsive to the comparison means to execute the handover algorithm.
A further object of the invention is an apparatus deciding on the handover in the mobile system, with which the method of the invention can be applied. The apparatus is characterized in that it is adapted to calculate the change in the level and/or quality of the signal between the mobile station and the serving base station and to execute the handover algorithm in response to a signal change at least equal to the predefined trigger value.
The invention is based on the idea that a signal change, such as an attenuation of a certain magnitude of the level and/or a drop of a certain magnitude of the quality, for instance the duplication of the bit error ratio, best indicates the need for handover.
Such a handover method provides the advantage that it immediately detects from a quick attenuation of the signal and/or a drop in quality, when handover is required and the most suitable cell can quickly be defined even in difficult environments and the handover can be executed without an extensive drop in the quality of the call.
The handover method of the invention further provides the advantage that the call can better be maintained with a quick handover to the most suitable base station and, at the same time, the number of handovers can be minimized. Another advantage of the handover method of the invention is that it is not necessary to keep checking for a possible handover in situations, where the signal is quite poor, but the best the environment can offer.
In a preferred embodiment of the invention the handover algorithm triggered by the signal change is only executed for a certain predefined period of time. This provides the advantage that the execution of a triggered handover algorithm ends at a certain point of time, even if the channel is not changed. Another advantage is that the handover algorithm is executed long enough to ascertain the necessity of the handover.
In a preferred embodiment of the invention one and the same operator-specified and normally applied handover algorithm is used in an accelerated mode for a certain time after it has been triggered. This provides the advantage that only one algorithm is required and, by simply changing the parameter values, it can be changed to a faster and more real-time algorithm suitable for a difficult environment.
The preferred embodiments of the method, system and apparatus of the invention are described in the appended dependent claims 2, 3, 4, 6, 7 and 9.